National Primary Care Research and Development Centre, University of Manchester, Manchester M13 9PL

National Primary Care Research and Development Centre, Centre for Health Economics, University of York, York YO1 5DD

Correspondence to: Professor Roland

Trials of healthcare interventions are often described as either explanatory or pragmatic. Explanatory trials generally measure efficacy-the benefit a treatment produces under ideal conditions, often using carefully defined subjects in a research clinic. Pragmatic trials measure effectiveness-the benefit the treatment produces in routine clinical practice.

An explanatory approach recruits as homogeneous a population as possible and aims primarily to further scientific knowledge. By contrast, the design of a pragmatic trial reflects variations between patients that occur in real clinical practice and aims to inform choices between treatments. To ensure generalisability pragmatic trials should, so far as possible, represent the patients to whom the treatment will be applied. The need for purchasers and providers of health care to use …

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